Photovoltaic cell with copper poor CIGS absorber layer and method of making thereof

ABSTRACT

A photovoltaic cell includes a p-type copper-indium-gallium-selenide absorber layer, where a content of Cu, In, and Ga in a first portion of the p-type copper-indium-gallium-selenide absorber layer satisfies the equation Cu/(In+Ga)≦0.3, and where the content is measured in atomic percent.

FIELD

The present invention is directed to sputtering targets and more particularly to methods of making the targets and using the targets to make copper-indium-gallium-selenide solar cells.

BACKGROUND

Copper indium diselenide (CuInSe₂, or CIS) and its higher band gap variants copper indium gallium diselenide (Cu(In,Ga)Se₂, or CIGS), copper indium aluminum diselenide (Cu(In,Al)Se₂), copper indium gallium aluminum diselenide (Cu(In,Ga,Al)Se₂) and any of these compounds with sulfur replacing some of the selenium represent a group of materials, referred to as copper indium selenide CIS based alloys, have desirable properties for use as the absorber layer in thin-film solar cells. To function as a solar absorber layer, these materials should be p-type semiconductors. This may be accomplished by establishing a slight deficiency in copper, while maintaining a chalcopyrite crystalline structure. In CIGS, gallium usually replaces 20% to 30% of the normal indium content to raise the band gap; however, there are significant and useful variations outside of this range. If gallium is replaced by aluminum, smaller amounts of aluminum are used to achieve the same band gap.

SUMMARY

An embodiment relates to a photovoltaic cell which includes a p-type copper-indium-gallium-selenide absorber layer, where a content of Cu, In, and Ga in a first portion of the p-type copper-indium-gallium-selenide absorber layer satisfies the equation Cu/(In+Ga)≦0.3, and where the content is measured in atomic percent. The p-type copper-indium-gallium-selenide absorber layer has a density greater than 80%.

Another embodiment relates to a method of making a sputtering target comprising a copper-indium-gallium alloy, where a non-zero content of copper, indium and gallium satisfies the equation Cu/(In+Ga)≦0.3, where the content is measured in atomic percent, and at least one of a sulfur compound, a sodium compound or a sulfur and sodium compound.

Another embodiment relates to a method of making a photovoltaic cell. The method includes depositing a first electrode layer over a substrate, depositing a first p-type copper-indium-gallium-selenide absorber layer over the first electrode, depositing a second copper-indium-gallium-selenide absorber layer over the first copper-indium-gallium-selenide absorber layer, depositing an n-type semiconductor layer over the second copper-indium-gallium-selenide absorber layer, and depositing a second electrode layer over the n-type semiconductor layer. A content of the Cu, In, and Ga in the first copper-indium-gallium-selenide absorber layer satisfies the equation Cu/(In+Ga)≧0.6, and a content of the Cu, In, and Ga in the second copper-indium-gallium-selenide absorber layer satisfies the equation Cu/(In+Ga)≦0.3. The content is measured in atomic percent. The second absorber layer has a density greater than 80%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a highly simplified schematic diagram of a top view of a sputtering apparatus that can be used for forming a compound semiconductor layer comprising copper, indium, gallium, selenium and sodium.

FIG. 2 is a schematic side cross-sectional view of a CIGS based solar cell according to one embodiment of the invention.

FIG. 3 illustrates a highly simplified schematic diagram of a top view of a modular sputtering apparatus that can be used to manufacture the solar cell depicted in FIG. 2.

FIG. 4 illustrates schematically the use of three sets of dual magnetrons to increase the deposition rate and grade the composition of the CIGS layer to vary its band gap.

FIG. 5 is a schematic side cross-sectional view of a CIGS based solar cell with a graded copper concentration according to an embodiment of the invention.

DETAILED DESCRIPTION

As grown copper indium diselenide based films are intrinsically p-type. However, it was found that a small amount of sodium dopants in copper indium diselenide based films (e.g. copper indium gallium diselenide films) increases the p-type conductivity of the copper indium gallium diselenide film and the open circuit voltage, and in turn, improves the efficiency of the solar cell. For example, Ramanathan (Ramanathan et al., Prog. Photovolt. Res. Appl. 11 (2003) 225, incorporated herein by reference in its entirety) teaches that a solar cell, having an efficiency as high as 19.5%, may be obtained by using a soda-lime glass substrate in combination with depositing a copper indium diselenide film under a high growth temperature. This method significantly improves the efficiency of a traditional solar cell by diffusing sodium from the glass substrate into the copper indium diselenide film. However, it is difficult to control the amount of the sodium provided to the copper indium diselenide based film and the speed of the sodium diffusion from a glass substrate. Furthermore, unlike glass substrates, other substrates, such as metal and plastic substrates, do not provide such a readily available supply of sodium.

Rudmann (Rudmann et al., Thin Solid Films 32 (2003) 37) teaches forming a NaF or NaSe layer between the copper indium gallium diselenide layer and a first electrode (also referred as a back electrode). Sodium doping in this system can be controlled by modulating the sodium diffusion from the NaF or NaSe layer into the copper indium gallium diselenide layer. Although the amount of sodium in copper indium gallium diselenide may be more controllable than in the Ramanathan method, the NaF or NaSe interlayer results in a poor adhesion and a greater schottky barrier between the copper indium gallium diselenide layer and the first electrode.

Yun (Yun et al., Thin Solid Films 515(2007) 5876-5879) teaches DC sputtering a sodium-containing molybdenum layer from a composite Na—Mo target. It has been found that resulting solar cells have an improved efficiency because the sodium incorporation enhances the open circuit voltage and fill factor. However, this method is limited by the property of the Na—Mo target, which has a high propensity to crack.

Embodiments of the present invention include sputtering targets which are useful in the fabrication of photovoltaic devices, methods of making photovoltaic devices with these targets and the photovoltaic devices themselves. One embodiment is a sputtering target which includes a sodium containing compound in a copper-indium-gallium matrix. The sputtering target of this embodiment may be used in the fabrication of a p-type copper indium gallium sodium diselenide layer of a photovoltaic device. Another embodiment includes sputtering targets which include a sulfur containing compound. This embodiment includes targets with indium, copper-indium, or copper-indium-gallium matrices. The sputtering targets of this embodiment may also be used in the fabrication of a p-type copper indium gallium sulfur diselenide layer of a photovoltaic device. In an alternative embodiment, both sodium and sulfur containing compounds can be added to the same sputtering target. As in the previous embodiment, the sputtering target of this embodiment may be used to form a p-type copper indium gallium sodium sulfur diselenide layer of a photovoltaic device.

Another embodiment is drawn to the use of a copper poor copper-indium-gallium sputtering target(s) at the end of a row of copper-indium-gallium sputtering targets to make a copper poor p-type copper-indium-gallium-selenium layer. However, unlike the prior art slightly copper deficient p-type copper-indium-gallium-selenium layers, the copper poor p-type copper-indium-gallium-selenium layer of the present embodiment satisfies the equation Cu/(In+Ga)≦0.3 in atomic percent. Further, sodium and/or sulfur can be added to the copper poor CIGS layer of this embodiment by using the sputtering targets of any of the previous embodiments.

Still another embodiment is a method of making an n-type window layer of a photovoltaic device by sputtering with a target which includes a sulfur containing compound. The n-type window layers of this embodiment include copper-indium-sulfide, and copper-indium-selenium-sulfide. An aspect of this embodiment is an n-type window layer which includes multiple sublayers in which the concentration of sulfur in each successive sublayer is greater than in the previous sublayer.

A first embodiment of the present invention is a sputtering target which includes a sodium compound and at least one metal selected from of copper, indium and gallium. The target may include the sodium containing compound and one of indium, copper-indium, copper-gallium or copper-indium-gallium (i.e., CIG). The copper, indium and gallium may be in the form of an alloy while the sodium containing compound may be in solution in the alloy or in the alloy as inclusion regions. Preferably, the content of the sodium compound in the target is between 0.1 and 5 weight percent while the content of copper, indium, and gallium is between 95 and 99.9 weight percent. The sputtering target may, however, include small amounts of impurities that either do not materially affect or do not negatively affect the performance of CIGS(Na) layers deposited on a substrate by a sputtering process which uses the sputtering target. In an embodiment, the as-formed target may include small amounts of selenium, aluminum, sulfur or group VII or VIII elements present in a sodium salt.

FIG. 1 illustrates a sputtering apparatus that can be used for forming a compound semiconductor layer with the sputtering target of the first embodiment. One or more targets comprising copper, indium, gallium, and a sodium containing compound (e.g., targets 27 c 1 and 27 c 2) are located in a sputtering process module 22 b, such as a vacuum chamber. While two targets are shown in FIG. 1, 1 to 8 targets may be used (e.g., six targets as shown in FIG. 4). The module 22 b may include a pumping device 23 to provide the desired vacuum. In this non-limiting example, the targets 27 c 1 and 27 c 2 are powered by DC power sources 7. A CIGS layer may be deposited over a substrate 100, such as a moving web substrate, by reactively sputtering the layer from targets 27 c 1, 27 c 2 in an atmosphere that comprises a sputtering gas (e.g., argon gas) and a selenium-containing gas, such as evaporated selenium or hydrogen selenide gas. Alternatively, the sputtering may comprise reactively alternating current (AC) magnetron sputtering the compound semiconductor layer from at least one pair of two conductive targets 27 c 1 and 27 c 2, in a sputtering atmosphere that comprises argon gas and a selenium-containing gas. In some embodiments, the conductive targets 27 c 1 and 27 c 2 comprise the same target materials. For example, each of the at least two conductive targets 27 c 1 and 27 c 2 may comprise copper, indium, gallium and a sodium containing compound, or copper, indium, aluminum and a sodium containing compound. The sputtering target, for example, may have a composition of about 29-41 wt % copper, including 29-39 wt % Cu, about 36-62 wt % indium, including 49-62 wt % In, about 8-25 wt % gallium, including 8-16 wt % Ga, and about 0.1 and 5 wt % sodium containing compound. In other embodiments, targets 27 c 1 and 27 c 2 may comprise different materials from each other as will be described below. Radiation heaters 30 maintain the substrate 100 at the required process temperature, for example, around 400-800° C., for example around 500-700° C., which is preferable for the CIS based alloy deposition.

In an embodiment, the sputtering targets 27 c 1, 27 c 2 may have a cylindrical shape. The sputtering targets 27 c 1, 27 c 2 may be a cylindrical shell formed on a cylindrical sputtering target support (e.g., a backing tube, such as a stainless steel, titanium, aluminum or other non-magnetic backing tube). Alternatively, the targets may comprise a hollow cylinder or hollow rings ring made without the backing tube. The cylinder or rings may be made in a mold or on a temporary support and then attached to the backing tube which houses the magnet(s), as described for example in U.S. application Ser. No. 12/588,578, filed on Oct. 20, 2009, and incorporated herein by reference in its entirety. Alternatively, planar targets may be used instead.

The sodium containing compound may be a sodium salt, such as a halogen salt or a chalcogen salt. Exemplary sodium containing compounds include, but are not limited to, sodium fluoride, sodium chloride, sodium molybdate, sodium selenide, sodium hydroxide, sodium oxide, sodium sulfate, sodium selenate, sodium sulfide, sodium sulfite, sodium tungstate, sodium titanate, sodium metavanadate, and sodium orthovanadate. As noted above, the salt may contain a transition metal in addition to the halogen or chalcogen. Example transition metals that may be contained in the salt include Mo, Ti, W, etc.

Another embodiment of the present invention is a method of making a sputtering target of copper, indium, gallium and sodium compound containing material. Exemplary methods include casting, injection molding, twin arc wire spraying (TWAS), plasma spraying and powder metallurgy.

Examples of casting processes (e.g., dip casting, vacuum mold casting, semi-solid casting, direct strip casting, centrifugal casting, continuous casting, squeeze casting, etc.) and injection molding processes are described in U.S. application Ser. No. 12/588,578, filed on Oct. 20, 2009, and incorporated herein by reference in its entirety. In the casting and injection molding processes, the copper, indium, gallium and sodium containing compound may be combined at a temperature high enough to form a liquid melt. For example, a sodium compound, such as a sodium salt, is added to a copper, indium and gallium containing melt to form a liquid sputtering material. The liquid is then solidified onto the target support which may be located in a mold or in other suitable casting or molding liquid receiving positions.

Alternatively, the copper, indium, gallium and sodium containing compound may be combined at a temperature suitable for forming a thixotropic slurry. Thixotropic metal melts as used herein are those in which the viscosity of the melt is lowered by mixing as the melt cools. Instead of forming interlocking dendrites on cooling, the precipitating solids have a more rounded, spheroidal shape allowing the melt to flow even at temperatures at which it would otherwise be semisolid. In one aspect, the copper, indium and gallium are combined at an appropriate thixotropic temperature in a container (e.g., melting chamber or melting pot) and then the sodium containing compound is added to this thixotropic slurry. Preferably, the thixotropic slurry is stirred by a stirrer or fin located in the chamber or pot prior to casting or injection molding.

Further, as discussed above, the sputtering target may be either formed on a target support or a hollow cylinder or ring made without a backing tube. Thus, embodiments of the present invention include casting and injection molding both supported sputtering targets (i.e., cast or molded onto the backing tube) and unsupported targets (not cast or molded with the backing tube as part of the casting mold).

As with the casting and injection molding embodiments, the TWAS method may be used to form both supported sputtering targets and unsupported. Additionally, the TWAS method may use compound wires. That is, wires that have a distinct core and shell made of different materials. In one embodiment, the core is made of a copper indium gallium alloy that includes a sodium containing compound. The surrounding shell may be made of a copper indium gallium alloy which contains no sodium or less sodium than the wire core. Alternatively, the shell is made of a copper indium gallium alloy that includes a sodium containing compound and the core is made of a copper indium gallium alloy which contains no sodium or less sodium than the shell.

Powder metallurgy processes include both hot pressing and cold pressing. If desired, the sodium salt may be obtained as a fine powder or crushed to a fine powder under a controlled atmosphere (e.g., crushing in an inert gas, such as argon or nitrogen, filled glovebox). In either process, the starting material may be powders of the individual copper, indium and gallium elements and the sodium containing compound. However, gallium is a liquid at room temperature. Therefore, the starting materials may preferably include alloys of gallium such as InGa, CuGa, and CuInGa (i.e., a copper, indium and gallium alloy). In another example, a copper powder, indium powder and copper-gallium alloy powder may be mixed with a sodium salt powder, preferably under the above described control atmosphere.

The starting materials may be mixed in the desired ratios and then pressed to form an unsupported sputtering target (i.e., pressed onto a temporary support and then attached to the backing tube) or pressed onto a sputtering support (e.g., backing tube). The pressing may be warm or cold, uniaxial or isostatic.

In another embodiment, the sodium containing compound is provided in solution. For example, a sodium salt, such as NaCl or NaF is mixed with a solvent, such as water to form a solution (e.g., salt water). In this embodiment, the powders of Ga, In and Cu (or their alloys) are mixed with the solution and then the solvent (e.g., water) is evaporated, leaving a sodium salt in the powder mixture. The evaporation may be conducted by heating the suspension comprising the salt solution and Cu, In and Ga containing (e.g., CIG) powder while agitating (e.g., tumbling or blending) the suspension. The heating may be conducted under vacuum or under forced air. The sodium salt may crystallize on the CIG powder particles. If the heating temperature is sufficiently low, then the salt may crystallize as a hydrate. Thus, a separate heating step may be conducted to dehydrate the salt.

Preferably, the sputtering targets made by the above methods exclude non-trace amounts of selenium prior to being used in a sputtering apparatus. Such targets may be advantageously used in a reactive sputtering process with an atmosphere that includes evaporated selenium or hydrogen selenide to form CIGS(Na) layers.

Another embodiment includes a method of making a photovoltaic device. The method includes a step of forming a compound semiconductor layer comprising copper, indium, gallium, selenium and sodium by reactive sputtering at least one first target comprising copper, indium, gallium and a sodium compound as described above in an atmosphere comprising selenium. The compound semiconductor layer may be a sodium doped first p-type copper indium gallium selenide absorber layer. The method may further include steps of forming a first electrode on a substrate below the first p-type copper indium gallium selenide absorber layer (i.e., the first electrode is formed prior to the p-CIGS(Na) layer with the p-CIGS(Na) being formed over the first electrode), forming an n-type window semiconductor layer over the first p-type copper indium gallium selenide absorber layer, and forming a transparent electrode over the n-type window semiconductor layer. In an embodiment, a p-n junction is formed between the n-type window semiconductor layer and the first p-type copper indium gallium selenide absorber layer. In an embodiment of the method, a web substrate may be moved though a plurality of sputtering chambers. In one aspect of this embodiment, the compound semiconductor layer is formed in one chamber of the plurality of chambers.

FIG. 3 illustrates a modular sputtering apparatus that may be used for depositing the layers for making a photovoltaic device. The apparatus is equipped with an input, or load, module 21 a and a symmetrical output, or unload, module 21 b. Between the input and output modules are process modules 22 a, 22 b, 22 c and 22 d. The number of process modules 22 may be varied to match the requirements of the device that is being produced. Each module has a pumping device 23, such as vacuum pump, for example a high throughput turbomolecular pump, to provide the required vacuum and to handle the flow of process gases during the sputtering operation. Each module may have a number of pumps placed at other locations selected to provide optimum pumping of process gases. The modules are connected together at slit valves 24, which contain very narrow low conductance isolation slots to prevent process gases from mixing between modules. These slots may be separately pumped if required to increase the isolation even further. Other module connectors 24 may also be used. Alternatively, a single large chamber may be internally segregated to effectively provide the module regions, if desired. U.S. Published Application No. 2005/0109392 A1 (“Hollars”), filed on Oct. 25, 2004, discloses a vacuum sputtering apparatus having connected modules, and is incorporated herein by reference in its entirety.

The web substrate 100 is moved throughout the machine by rollers 28, or other devices. Additional guide rollers may be used. Rollers shown in FIG. 3 are schematic and non-limiting examples. Some rollers may be bowed to spread the web, some may move to provide web steering, some may provide web tension feedback to servo controllers, and others may be mere idlers to run the web in desired positions. The input spool 31 a and optional output spool 31 b thus are actively driven and controlled by feedback signals to keep the web in constant tension throughout the machine. In addition, the input and output modules may each contain a web splicing region or device 29 where the web 100 can be cut and spliced to a leader or trailer section to facilitate loading and unloading of the roll. In some embodiments, the web 100, instead of being rolled up onto output spool 31 b, may be sliced into solar modules or cells by the web splicing device 29 in the output module 21 b. In these embodiments, the output spool 31 b may be omitted. As a non-limiting example, some of the devices/steps may be omitted or replaced by any other suitable devices/steps. For example, bowed rollers and/or steering rollers may be omitted in some embodiments.

Heater arrays 30 are placed in locations where necessary to provide web heating depending upon process requirements. These heaters 30 may be a matrix of high temperature quartz lamps laid out across the width of the web. Infrared sensors provide a feedback signal to servo the lamp power and provide uniform heating across the web. In one embodiment, as shown in FIG. 3, the heaters are placed on one side of the web 100, and sputtering targets 27 a-e are placed on the other side of the web 100. Sputtering targets 27 may be mounted on dual cylindrical rotary magnetron(s), or planar magnetron(s) sputtering sources, or RF sputtering sources.

After being pre-cleaned, the web substrate 100 may first pass by heater array 30 f in module 21 a, which provides at least enough heat to remove surface adsorbed water. Subsequently, the web can pass over roller 32, which can be a special roller configured as a cylindrical rotary magnetron. This allows the surface of electrically conducting (metallic) webs to be continuously cleaned by DC, AC, or RF sputtering as it passes around the roller/magnetron. The sputtered web material is caught on shield 33, which is periodically changed. Preferably, another roller/magnetron may be added (not shown) to clean the back surface of the web 100. Direct sputter cleaning of a web 100 will cause the same electrical bias to be present on the web throughout the machine, which, depending on the particular process involved, might be undesirable in other sections of the machine. The biasing can be avoided by sputter cleaning with linear ion guns instead of magnetrons, or the cleaning could be accomplished in a separate smaller machine prior to loading into this large roll coater. Also, a corona glow discharge treatment could be performed at this position without introducing an electrical bias.

Next, the web 100 passes into the process module 22 a through valve 24. Following the direction of the imaginary arrows along the web 100, the full stack of layers may be deposited in one continuous process. The first electrode 200 may be sputtered in the process module 22 a over the web 100, as illustrated in FIG. 3. Optionally, the process module 22 a may include more than one target (e.g., for sputtering barrier and/or adhesion layers).

The compound semiconductor layer may be made by (1) depositing the sodium doped first p-type copper indium gallium selenide absorber layer over the first electrode by sputtering a first target comprising copper, indium, gallium and the sodium containing compound (such as a sodium-transition metal-halogen or sodium-transition metal-chalcogen salt and (2) depositing a second p-type copper indium gallium selenide absorber layer over the sodium doped first p-type copper indium gallium selenide absorber layer by sputtering from a second target that comprises copper, indium, gallium and either no sodium or a lower amount of sodium than the at least one first target. Preferably, the second p-type copper indium gallium selenide absorber layer contains no sodium or a lower sodium content that the sodium doped first p-type copper indium gallium selenide absorber layer. If the sodium salt contains molybdenum (e.g., NaMoO₄, etc.), then the first p-type CIGS(Na) should be deposited on a first electrode that comprises Mo. Thus, in this embodiment, most or all of the sodium is provided into the bottom sublayer of the p-CIGS absorber. Since the bottom sublayer is located adjacent to a transition metal lower electrode, the sodium salt may include the same transition metal as the lower electrode.

Alternatively, most or all of the sodium may be provided from the upper portion or sublayer of the p-CIGS absorber. In this embodiment, the sodium salt preferably lacks the transition metal since the upper portion or sublayer is located adjacent to the p-n junction and distal from the lower electrode. Thus, the compound semiconductor layer may be formed by a method that include the steps of (1) depositing a first p-type copper indium gallium selenide absorber layer over the first electrode by sputtering from a target that comprises copper, indium, gallium and either no sodium or a lower amount of sodium than a successive target and (2) depositing a second sodium doped p-type copper indium gallium selenide absorber layer over the first p-type copper indium gallium selenide absorber layer by sputtering from a target comprising copper, indium, gallium and a higher amount of the sodium containing compound than in the first target. In this embodiment, the sodium containing compound preferably does not include molybdenum.

The substrate 100 may be a foil web, for example, a metal web substrate, a polymer web substrate, or a polymer coated metal web substrate, and may be continuously passing through the sputtering module 22 b during the sputtering process, following the direction of the imaginary arrow along the web 100. Any suitable materials may be used for the foil web. For example, metal (e.g., stainless steel, aluminum, or titanium) or thermally stable polymers (e.g., polyimide or the like) may be used. The foil web 100 may move at a constant or variable rate to enhance deposition.

The web 100 may then pass into the process modules 22 c and 22 d, for depositing the n-type semiconductor layer 302, and the transparent top electrode 400, respectively. Any suitable type of sputtering sources may be used, for example, rotating AC magnetrons, RF magnetrons, or planar magnetrons. Extra magnetron stations (not shown), or extra process modules (not shown) could be added for sputtering the optional one or more anti-reflection (AR) layers.

Finally, the web 100 passes into output module 21 b, where it is either wound onto the take up spool 31 b, or sliced into solar cells using cutting apparatus 29. While sputtering was described as the preferred method for depositing all layers onto the substrate, some layers may be deposited by MBE, CVD, evaporation, plating, etc., while, preferably, the CIS based alloy is reactively sputtered.

FIG. 2 illustrates a solar cell that may be made by embodiments of the methods discussed above. The solar cell contains the substrate 100 and a first (lower) electrode 200. The first electrode may comprise a transition metal, for example, one of Mo, W, Ta, V, Ti, Nb, Zr, or alloys thereof. In one embodiment, the step of depositing the first electrode 200 may comprise DC sputtering a first target 27 a comprising a transition metal, such as molybdenum. Optionally, the first electrode 200 of the solar cell may comprise one or more barrier layers (not shown) located at the bottom of the electrode 200, and/or one or more adhesion layers (not shown) located on top of the electrode 200. The barrier layer 201 substantially prevents alkali diffusion into the substrate 100. The optional barrier layer and adhesion layer may comprise any suitable materials. For example, they may be independently selected from a group consisting Mo, W, Ta, V, Ti, Nb, Zr, Cr, TiN, ZrN, TaN, VN, V₂N or combinations thereof.

Over the first electrode 200 is a p-type semiconductor absorber layer 301 comprising CIGS(Na). Layer 301 may have a stoichiometric composition having a Group I to Group III to Group VI atomic ratio of about 1:1:2, or a non-stoichiometric composition having an atomic ratio of other than about 1:1:2. Preferably, layer 301 is slightly copper deficient and has a slightly less than one copper atom (e.g., 0.8 to 0.99 copper atoms) for each one of Group III atom and each two of Group VI atoms. The step of depositing the at least one p-type semiconductor absorber layer may comprise reactively DC sputtering or reactively AC sputtering the semiconductor absorber layer from at least two electrically conductive targets 27 c 1, 27 c 2 in a sputtering atmosphere that comprises argon gas and a selenium containing gas (e.g. selenium vapor or hydrogen selenide as described with respect to FIG. 1). An n-type semiconductor layer 302 may then be deposited over the p-type semiconductor absorber layer 301. The n-type semiconductor layer 302 may comprise any suitable n-type semiconductor “window” materials, for example, but not limited to ZnS, ZnSe or CdS.

A second electrode 400, also referred to as a transparent top electrode, is further deposited over the n-type semiconductor layer 302. The transparent top electrode 400 may comprise multiple transparent conductive layers, for example, but not limited to, one or more of an Indium Tin Oxide (ITO), Zinc Oxide (ZnO) or Aluminum Zinc Oxide (AZO) layers 402 located over an optional resistive Aluminum Zinc Oxide (RAZO) layer 401. Of course, the transparent top electrode 400 may comprise any other suitable materials, for example, doped ZnO or SnO.

Optionally, one or more antireflection (AR) films (not shown) may be deposited over the transparent top electrode 400, to optimize the light absorption in the cell, and/or current collection grid lines may be deposited over the top conducting oxide.

Alternatively, the solar cell may be formed in reverse order. In this configuration, a transparent electrode is deposited over a substrate, followed by depositing an n-type semiconductor layer over the transparent electrode, depositing at least one p-type semiconductor absorber layer over the n-type semiconductor layer, and depositing a top electrode comprising an alkali-containing transition metal layer over the at least one p-type semiconductor absorber layer. The substrate may be a transparent substrate (e.g., glass) or opaque (e.g., metal). If the substrate used is opaque, then the initial substrate may be delaminated after the steps of depositing the stack of the above described layers, and then bonding a glass or other transparent substrate to the transparent electrode of the stack.

More preferably, the steps of depositing the first electrode 200, depositing the at least one p-type semiconductor absorber layer 301, depositing the n-type semiconductor layer 302, and depositing the second electrode 400 comprise sputtering in corresponding process modules of a plurality of independently isolated, connected process modules without breaking vacuum, while passing the web substrate 100 from an input module to an output module through the plurality of independently isolated, connected process modules such that the web substrate continuously extends from the input module to the output module while passing through the plurality of the independently isolated, connected process modules as shown in FIG. 3. Each of the process modules may include one or more sputtering targets for sputtering material over the web substrate 100.

In some embodiments, at least one p-type semiconductor absorber layer 301 may comprise graded CIS based material. In this embodiment, the process module 22 b further comprises at least two more pairs of targets (227, and 327), as illustrated in FIG. 4. The first magnetron pair 127 (27 c 1 and 27 c 2) are used to sputter a layer of copper indium diselenide while the next two pairs 227, 327 of magnetrons targets (27 c 3, 27 c 4 and 27 c 5, 27 c 6) sputter deposit layers with increasing amounts of gallium (or aluminum), thus increasing and grading the band gap. The total number of targets pairs may be varied, for example may be 2-10 pairs, such as 3-5 pairs. This will grade the band gap from about 1 eV at the bottom to about 1.3 eV near the top of the layer. Details of depositing the graded CIS based material is described in the Hollars published application, which is incorporated herein by reference in its entirety.

Optionally, one or more process modules (not shown) may be added between the process modules 21 a and 22 a to sputter a back side protective layer over the back side of the substrate 100 before the electrode 200 is deposited on the front side of the substrate. U.S. application Ser. No. 12/379,428 entitled “Protective Layer for Large-Scale Production of Thin-Film Solar Cells” and filed on Feb. 20, 2009, which is hereby incorporated by reference, describes such deposition process. Further, one or more barrier layers may be sputtered over the front side of the substrate 100 in the process module(s) added between the process modules 21 a and 22 a. Similarly, one or more process modules (not shown) may be added between the process modules 22 a and 22 b, to sputter one or more adhesion layers between the electrode 200 and the CIGS layer 301. Alternatively, the adhesion layers may be sputtered in the same chamber 22 a using a second, different sputtering target from target 27 a.

Another embodiment of the invention relates to a method of making a photovoltaic device in which the CIGS layer contains sulfur. This method is similar to the methods of making photovoltaic device in which the CIGS layer contains sodium discussed above. The method includes the step of forming a compound semiconductor layer that includes copper, indium, gallium, selenium and sulfur by reactive sputtering from a target that includes copper, indium, gallium and a sulfur compound in an atmosphere comprising selenium. Typically, the compound semiconductor layer formed by this method is a p-type copper indium gallium sulfur selenide absorber layer. The method may also include a steps of (1) forming the first electrode 200 below the p-type copper indium gallium sulfur selenide absorber layer 301 (that is, the first electrode is formed prior forming the p-type CIG(Se,S) absorber layer), (2) forming an n-type window semiconductor layer 302 over the p-type copper indium gallium sulfur selenide absorber layer 301 and (3) forming a transparent electrode 400 over the n-type window semiconductor layer as shown in FIG. 2. Because the absorber layer (p-type) and the window layer (n-type) have different doping, a p-n junction is formed between the window layer and the absorber layer.

As in the previous device fabrication embodiments discussed above, devices with sulfur containing CIGS layers may be made in the apparatus illustrated in FIG. 3. That is, the device may be made by moving a web substrate 100 though a plurality of sputtering chambers 22.

The p-type absorber layer 301 may be made by reactive sputtering from a copper indium gallium sulfur compound containing target in a selenium containing atmosphere. The n-type window semiconductor layer 302 may be formed by sputtering an n-CdS layer or by sputtering an n-type copper indium sulfur selenide layer or a copper indium sulfide layer by reactive or non-reactive sputtering, as will be discussed in more detail below.

The p-type absorber layer may include sodium in addition to sulfur. This p-type absorber layer may be made by sputtering from sputtering targets in which the sulfur and the sodium compounds are in the same sputtering target. Salts that include both sodium and sulfur include, but are not limited to, Na₂S and Na₂SO₃. Separate sodium and sulfur salts may also be used in the same sputtering target. Alternatively, a pair of sputtering targets may be used in which the sulfur compound is in one target and the sodium compound is in the other target.

Another embodiment of the invention relates to sputtering targets comprising a sulfur compound. In one aspect, the sputtering target comprises indium and a sulfur compound. Alternatively, the target may include indium and at least one of copper, gallium and selenium (e.g., a copper-indium-gallium alloy or a copper-indium alloy). Sulfur compounds that may be used to make sputtering targets of this embodiment include CuS, Cu₂S, Na₂S, Na₂SO₃, GaS, In₂S₃, Se₂S₆, S₇NH, and N₄S₃F₃. The sputtering targets preferably have a sulfur concentration in a range of 5 to 40 at %, such as 5-15 at %.

Another embodiment relates to a method of making a sputtering target containing sulfur. One embodiment of the method includes the steps of (1) combining an indium containing material and a sulfur containing compound to form a combined sputtering material and (2) providing the combined sputtering material onto a target support to form an indium and sulfur compound containing target. As with the sodium containing sputtering targets discussed above, unsupported targets may be made as well. Further, as with the sodium containing sputtering targets, the sulfur containing sputtering targets may be formed by casting, injection molding, twin arc wire spraying (TWAS), plasma spraying or powder metallurgy, as described above with respect to the sodium salt embodiment.

Another embodiment relates to a method of making a photovoltaic cell that has two adjacent p-type CIGS absorber layers or sublayers 301 a, 301 b in which the second (or top) p-type absorber layer 301 b that is located closest to the p-n junction is copper poor and has significantly less copper than the first or bottom p-type CIGS layer 301 a, as shown in FIG. 5. The photovoltaic cell shown in FIG. 5 includes a first electrode layer 200 over a substrate, the copper-indium-gallium-selenide absorber layers 301 a, 301 b over the first electrode, an n-type semiconductor layer 302 over the copper-indium-gallium-selenide absorber layers and a second electrode 400 layer over the n-type semiconductor layer.

The content of the Cu, In, and Ga in the second copper-indium-gallium-selenide absorber layer 301 b satisfies the equation Cu/(In+Ga)≦0.3, such as 0.05 to 0.3, such as 0.15-0.2 in atomic percent. The content of the Cu, In, and Ga in the first copper-indium-gallium-selenide absorber layer 301 a satisfies the equation Cu/(In+Ga)≧0.6-0.99, such as 0.8-0.99. All the contents are measured in atomic percent.

The method of making the cell shown in FIG. 5 includes the steps of (1) depositing a first electrode layer 200 over a substrate, (2) depositing the first p-type copper-indium-gallium-selenide absorber layer 301 a over the first electrode, (3) depositing the second copper-indium-gallium-selenide absorber layer 301 b over the first copper-indium-gallium-selenide absorber layer 301 a (4) depositing an n-type semiconductor layer 302 over the second copper-indium-gallium-selenide absorber layer 301 b and (5) depositing a second electrode layer over the n-type semiconductor layer 400. Both the first and second copper-indium gallium-selenide absorber layers may deposited by reactive sputtering a copper-indium gallium sputtering target(s) in a selenium containing ambient.

As in the previous embodiments, devices according to this embodiment may be made with the apparatus of FIGS. 3 and 4. That is, the devices may be made by moving a web substrate through a plurality of sputtering chambers. The first and second copper-indium-gallium-selenide absorber layers 301 a, 301 b may be formed in the same or different sputtering chambers. Further, the first and second p-type absorber layers may be deposited from different targets, with the target(s) 27 c 5 and 27 c 6 used to deposit the second layer 301 b having less copper then the target(s) 27 c 1-27 c 4 used to deposit the first layer 301 a.

For example, as shown in FIG. 4, the first pair 127 of targets 27 c 1 and 27 c 2, and the second pair 227 of targets 27 c 3 and 27 c 4 may comprise slightly copper deficient CIG targets which are used to deposit the first p-CIGS layer 301 a which is slightly copper deficient. Targets 27 c 1-27 c 4 may contain about 29-41 wt % copper, about 36-62 wt % indium, and about 8-25 wt % gallium.

The third pair 327 of targets 27 c 5 and 27 c 6 may comprise copper poor CIG targets which have less copper than targets 27 c 1-27 c 4. Targets 27 c 5 and 27 c 6 are used to deposit the second p-CIGS layer 301 b which is copper poor. The targets 27 c 5 and 27 c 6 preferably contain about 8-15 wt % copper, about 55-80 wt % indium, and about 10-25 wt % gallium. The Cu to Ga ratio in the second copper-indium-gallium-selenide absorber layer 301 b and in targets 27 c 5 and 27 c 6 may vary from 0.4 to 0.6. Further, the Cu:III (i.e., Cu:In+Ga) ratio may be <0.3, such as 0.05-0.3, or 0.15-0.2.

Further, the Cu content may be graded along the thickness of the first 301 a and the second 301 b p-CIGS layers. To form the graded Cu content, the amount of Cu in each pair 127, 227, 327 of targets is different from the other pair(s), as described above.

In one embodiment, the copper poor last sputtering target(s) (e.g., target pair 327) in a series of sputtering targets (e.g., pairs 127, 227 and 327) includes a sulfur containing compound, such as the sulfur containing sputtering targets discussed above. In another embodiment, the copper poor last sputtering target(s) (e.g., target pair 327) in a series of sputtering targets includes a sodium containing compound, such as the sodium containing sputtering targets discussed above. Including sodium in the upper most layer 301 b allows the bottom CIGS layer 301 a to have an improved grain growth (i.e., larger grains) without presence of sodium. If desired, the targets 27 c 5 and 27 c 6 may be made by casting at lower melt temperature than targets 27 c 1-27 c 4 due to a lower Cu to In and Ga ratio in targets 27 c 5 and 27 c 6 compared to targets 27 c 1-27 c 4. The lower melt temperature enhances the integration of the sodium salt into the melt and into the resulting target. Alternatively, the last sputtering target(s) (e.g., target pair 327) in a series of sputtering targets may include both a sulfur containing compound and a sodium containing compound.

In still another embodiment, the first copper-indium-gallium-selenide absorber layer 301 a is deposited at a higher temperature than the second copper-indium-gallium selenide absorber layer 301 b. Thus, a lower amount of sodium may be required in the top layer 301 b since sodium would have less time to evaporate because the hotter bottom layer 301 a deposition with enhanced grain growth occurs before sodium is provided into the device. In an embodiment, the second copper-indium-gallium-selenide absorber layer 301 b may be sputtered at a substrate temperature≧600° C. For example, sputtering may be performed at a substrate temperature of 600-700° C. The first copper-indium-gallium-selenide absorber layer 301 a may be deposited at a substrate temperature<600° C., such as 400-575° C., e.g., 500-575° C. Preferably, the second copper-indium-gallium-selenide absorber layer 301 b has a density greater than 80%, such as 80-95%.

Another embodiment relates to the photovoltaic device having a p-CIGS absorber 301 and a sulfur containing n-CIS type window layer 302. A method of making the sulfur containing n-type window layer of the photovoltaic device includes sputtering with a target which includes a sulfur containing compound. The CIS type n-type window layer of this embodiment includes copper-indium-sulfide, and copper-indium-selenium-sulfide. Preferably, gallium is excluded from the target and the n-CIS type window layer 302. For example, the p-type CIGS layer 301 is copper deficient, while the n-type CIGS layer 302 contains sulfur. Therefore, while the layers 301 and 302 have a different composition, they are both CIS type layers which allows the p-n junction to be similar to a homojunction rather than a heterojunction.

An aspect of this embodiment is an n-type window layer which includes multiple sublayers in which the concentration of sulfur in each successive sublayer is greater than in the previous sublayer as illustrated in FIG. 5. The method includes steps of providing a substrate, depositing a first electrode layer 200 over the substrate 100, sputter depositing a first compound semiconductor layer 301 comprising copper, indium, gallium and selenium over the first electrode layer 200, sputter depositing a second compound semiconductor layer 302 a 1302 b comprising copper, indium and sulfur over the first compound semiconductor layer 301 and depositing a second electrode 400 over the second compound semiconductor layer, as shown in FIG. 3 for example. In one aspect, the second compound semiconductor layer 302 has a higher bandgap than the first compound semiconductor layer 301.

The second compound semiconductor layer 302 a/302 b may be sputter deposited from a sputtering target comprising sulfur, copper and indium, such as the sputtering targets discussed in more detail above. The sputtering target may further comprise selenium and/or selenium may be present in ambient. The sulfur containing sputtering target may contain a sulfur compound, such as a sulfur salt, as described in the previous embodiment directed to sulfur salt containing targets.

The second compound semiconductor layer 302 a/302 b may comprise a series of compound semiconductor sublayers 302 a, 302 b of copper, indium, and sulfur. Preferably, the concentration of sulfur in each successive sublayer 302 a, 302 b of the series of sublayers 302 a, 302 b is greater than in the preceding sublayer 302 a, 302 b.

The second compound semiconductor layer 302 a/302 b may be formed by sequentially passing the substrate 100 past a series of targets (27 c 1-27 c 6 in FIG. 4) in which each successive target contains more sulfur than the preceding target. The content of sulfur in the second compound semiconductor layer may be 5-40 at %, such as 5-15 at % while the copper to indium ratio may be 0.7-0.95 at %. In an embodiment, the content of chalcogen (e.g., S and/or Se) in layer 302 a/302 b may be about 50 at % while the content of metal may be about 50%. Deposition of the first compound semiconductor layer may be performed in the first chamber by reactive sputtering copper indium gallium selenide from one or more copper-indium-gallium alloy targets in a selenium containing ambient, and deposition of the second compound semiconductor layer may be performed in a second chamber by non-reactive sputtering from the at least one target comprising copper-indium-sulfur, or copper-indium-sulfur-selenium or by reactive sputtering in a selenium containing ambient. When performing non-reactive sputtering, the sputtering target may have a sulfur content of 5-40 at %, such as 5-15%, and a Se content of 10-45 at %, such as 35-45 at % to make a content of chalcogen of about 50 at % and a content of metal of about 50% in the semiconductor layer. If the semiconductor layer lacks Se, then it may comprise up to 50 at % S. The semiconductor layer preferably has a copper:indium ratio of 0.7-0.95 at %.

Each layer or method step or target from each embodiment described above may be used in any suitable combination with one or more layers, method steps or targets from anyone or more other embodiments described above. Thus, the method may include forming a p-CIGS layer containing both sodium and sulfur by sputtering one or more sodium and/or sulfur salt containing targets, forming a copper poor upper portion of the p-CIGS layer (with or without sodium or sulfur) and forming a sulfur containing n-CIGS window layer.

Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety. 

What is claimed is:
 1. A method of making a photovoltaic cell comprising: depositing a first electrode layer over a substrate; depositing a first p-type copper-indium-gallium-selenide absorber layer over the first electrode; depositing a second copper-indium-gallium-selenide absorber layer over the first copper-indium-gallium-selenide absorber layer, wherein the second absorber layer has less copper than the first p-type copper-indium-gallium-selenide absorber layer depositing an n-type semiconductor layer over the second copper-indium-gallium-selenide absorber layer; and depositing a second electrode layer over the n-type semiconductor layer, wherein a content of the Cu, In, and Ga in the first copper-indium-gallium-selenide absorber layer satisfies the equation Cu/(In+Ga)≧0.6, the first copper-indium-gallium-selenide absorber layer is deposited using a first copper-indium-gallium sputtering target, and the second copper-indium-gallium-selenide absorber layer is deposited using a second copper-indium-gallium sputtering target in which a content of the Cu, In, and Ga in the second copper-indium-gallium-selenide sputtering target satisfies the equation Cu/(In+Ga)≦0.3, wherein the content is measured in atomic percent.
 2. The method of claim 1, wherein the first and second copper-indium-gallium-selenide absorber layers are deposited by sputtering in a selenium containing ambient.
 3. The method of claim 2, wherein the sputtering of the second absorber layer is performed at a substrate temperature≧600° C.
 4. The method claim 3, wherein the sputtering of the second absorber layer is performed at a substrate temperature of 600-700° C.
 5. The method of claim 2, wherein depositing is performed with a series of sputtering targets and a last sputtering target in the series of sputtering targets includes a sulfur containing compound.
 6. The method of claim 5, wherein the sulfur compound comprises one or more of copper sulfide or sodium sulfide.
 7. The method of claim 5, wherein at least one sputtering target in a series of sputtering targets includes a sodium containing compound.
 8. The method of claim 7, wherein the sodium containing compound comprises one or more of sodium fluoride, sodium molybdate, sodium selenide, sodium hydroxide, sodium oxide, sodium sulfate, sodium selenate, sodium sulfide, sodium sulfite, sodium titanate, sodium metavanadate, or sodium orthovanadate.
 9. The method of claim 2, wherein the last sputtering target in a series of sputtering targets includes a sulfur containing compound and a sodium containing compound.
 10. The method of claim 1, wherein the Cu content is graded along a thickness of the compound semiconductor layer.
 11. The method of claim 1 further comprising: moving a web substrate through a plurality of sputtering chambers; and forming the first and second copper-indium-gallium-selenide absorber layers in the same or different sputtering chambers.
 12. The method of claim 11, wherein: depositing the first copper-indium-gallium-selenide absorber layer comprises sputtering from a plurality of first sputtering targets each comprising copper, indium and gallium; and depositing the second copper-indium-gallium-selenide absorber layer comprises sputtering from a plurality of second sputtering targets each comprising copper, indium and gallium, wherein the at least one second target contains less copper than the at least one first target.
 13. The method of claim 12, wherein the at least one second target contains a sodium salt or a sulfur compound.
 14. The method of claim 11, wherein the first copper-indium-gallium-selenide absorber layer is deposited at a lower temperature than the second copper-indium-gallium-selenide absorber layer.
 15. The method of claim 1, wherein the first copper-indium-gallium sputtering target contains about 29-41 wt % copper, about 36-62 wt % indium, and about 8-25 wt % gallium, and the second copper-indium-gallium sputtering target contains about 8-15 wt % copper, about 55-80 wt % indium, and about 10-25 wt % gallium. 